The present invention relates to a seat belt retractor and particularly to a locking mechanism for a seat belt retractor.
A seat belt retractor typically comprises a spool on which seat belt webbing is wound and which is mounted in a frame to rotate about its longitudinal axis to pay out or wind in webbing as required by movement of the vehicle occupant. In the event of a crash the spool is locked against rotation and thus payout is prevented and the vehicle occupant is securely restrained against forward motion.
The spool is usually locked by engagement of ratchet teeth at its ends with at least one toothed pawl mounted in a load-bearing manner to the frame (e.g., on a lockbar extending across the width of the spool).
The locking pawl is under considerable strain during a crash and must be constructed to be very strong. It is not unusual for the teeth on the locking pawl to shear under particularly high crash forces.
According to the present invention there is provided a seat belt retractor comprising a rotatable spool mounted in a frame, a primary locking mechanism for arresting rotation of the spool and a load absorbing mechanism arranged to come into effect at a predetermined load for absorbing a portion of the spool locking load.
According to a preferred embodiment of the invention, the load absorbing mechanism comprises a section of an inwardly facing peripheral edge of the frame having a serrated or roughened texture and which is positioned and adapted so that above the predetermined load it co-operates with a smooth surfaced spool bearing face to absorb some of the load.
Spool rotation is prevented on engagement of the primary locking mechanism and the load on the primary mechanism rises causing the frame side walls to deform and the serrated or roughened section is pushed closer to, and eventually against the spool bearing face, roughening the smooth surface and increasing its coefficient of. friction to absorb some of the load and resist rotation of the spool.
The load absorbing mechanism does not effect locking of the seat belt retractor but instead effectively shares the load which would otherwise be placed on the locking pawl and the spool and may be used to spread the load around at least part of the periphery of the frame aperture.
In consequence, the seat belt retractor can take greater loads than was hitherto possible and/or can make use of lighter and less costly materials.
In some known seat belt retractors, a secondary locking arrangement is used to lock the spool with the frame by means of complementary locking formations, (e.g., inter-engaging teeth). This arrangement has disadvantages because the loads are very high and the teeth can shear if the locking is not suitably phased. No such phasing is necessary for the arrangement of the present invention.